An important concern during microelectronic manufacturing is minimization of particulate contamination. Efforts have been made to reduce particulate contamination resulting from exposure during the transfer of a microelectronic substrate into a processing chamber. One area of development centers on a load lock, which is a vessel that operates in conjunction with a process chamber for microelectronic substrate processing. The load lock provides an intermediary for transferring a substrate from normal atmospheric pressure to the evacuated processing chamber.
Conventional load locks for vacuum processing chambers typically include a door for substrate loading and unloading under atmospheric pressure. Small particulates generated by moving parts within the load lock are typically suspended in the load lock while the vessel is at atmospheric pressure. Gravity has little effect on such small particulates. The particulates exist throughout the entire volume of the conventional load lock and may attach themselves to substrates. The particulates can interfere with substrate processing and reduce the yield of the operation.
U.S. Pat. No. 4,632,624 to Mirkovich et al. entitled Vacuum Load Lock Apparatus and U.S. Pat. No. 4,727,993 to Mirkovich et al. entitled Wafer Cassette Having Multi-Directional Access describe load locks which reduce wafer exposure to particulates during processing. Both patents disclose an upper and lower bell jar, the lower bell jar being removable from the assembly for the disposal of broken wafer pieces. The lower removable bell jar permits disposal of particulates to reduce particulate contamination. Unfortunately, many particulates are suspended in the load lock air and are unaffected by gravity, so that the removal of the lower bell jar does not remove these particulates.
U.S. Pat. No. 4,687,542 to Davis et al. entitled Vacuum Processing System and U.S. Pat. No. 4,842,680 to Davis et al. entitled Advanced Vacuum Processor both disclose a vacuum processing system including a load lock. Once the wafers have been inserted into the load lock and the load lock door is closed, a high pressure purge is applied through a manifold located on the inside of the load lock door. This high pressure purge provides a vertical flow to displace particles in a downward direction and also helps to blow off some of the large particles which have collected on the wafer carrier during its exposure to atmospheric conditions. The particles come to rest on the bottom of the load lock chamber. The chamber is then slowly evacuated in order not to aggravate the particles displaced on the bottom of the chamber.
The Davis et al. patents do not remove particulate contaminants from the load lock, but rather rely upon forcing the particulates to the bottom of the chamber and settling of particulates at the bottom of the chamber. Slow evacuation and lack of movement must occur in order not to disturb particulates on the chamber bottom. Unfortunately, some disturbance will always occur. Moreover, even without a disturbance, lightweight particulates will not all settle on the bottom, or remain on the bottom. Accordingly, only incomplete removal of particulates is accomplished.